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Monkey Tool Time

Erika FleuryDecember 18, 2013

It was once widely believed humans were the only animals to use tools. But then Jane Goodall and other scientists noticed chimpanzees using sticks to fish for termites and rocks to break open nuts. Now, many different types of animals have been observed using tools — dolphins, octopuses, blue jays, crows, fish, and sea otters, among others.

In the laboratory, capuchins have also proven to be adept at sequential tool use: using one tool to obtain a second tool that can then be used to obtain an out-of-reach goal. Only a few primates and corvids have demonstrated success at sequential tool use. It’s most often tested in the laboratory by offering the animal a food item that is out of reach and a tool that is not long enough to reach the food but is long enough to reach a second tool, which is long enough to reach the food. Animals succeed when they use the shorter tool to grab the longer tool, then use the longer tool to obtain the food.

Researchers believe sequential tool use requires a greater degree of cognitive sophistication than simply using one tool because the tool user has to 1) recognize a tool can be used on an item other than food, 2) resist the impulse to use the first tool to try to reach the food directly, and 3) organize his or her behavior hierarchically.

Step 1, Step 2, Yogurt

Although this experimental paradigm is common, it can’t disentangle all the cognitive processes involved in sequential tool use. So Gloria Sabbatini and her colleagues designed a new task to control for some of the limitations in the typical sequential tool use experiments.

The researchers presented 10 tufted capuchin monkeys with a problem. On one side of the monkey was a 90° angled Plexiglass tube filled with yogurt and on the other side was a platform, both out of reach. An experimenter showed the monkey a tool (either a rigid rod or a flexible one) and demonstrated how bendy it was before placing it on the platform just out of reach. Then the experimenter handed the monkey another tool (again, either a rigid or flexible rod). To reach the yogurt, the monkey had to insert the flexible tool inside the tube. If the flexible tool was out of reach on the platform, the monkey had to use the rigid tool to rake in the flexible tool and then use it to get the yogurt.

All the previous studies used tools that varied in a visual property (length or size), whereas the two tools in this study differed in a nonvisual property: rigidity. And in some previous studies the unreachable tool was always the correct tool for the task, so the animal’s only correct option was to use the first tool to reach the second one. This new study included conditions in which using the second tool wasn’t necessary to reach the food (i.e., when the monkey was handed the flexible tool) as well as conditions in which the unreachable tool was sometimes inappropriate for the task (when the second tool on the platform was the rigid tool).

The capuchins passed this new sequential tool use task with flying colors. When they were handed a rigid tool and faced with an out-of-reach flexible tool, the monkeys used the rigid tool to rake in the flexible tool and then used the flexible tool to reach the yogurt. Nine of the 10 capuchins accomplished this on the very first trial.

If they were handed the flexible tool, all 10 capuchins simply used it to reach the yogurt, not bothering with the second, unnecessary tool on the platform.

When presented with the rigid tool and another rigid tool on the platform, the capuchins were just as likely to try to probe the yogurt tube with the tool handed to them, retrieve the platform tool, or discard the rigid tool they had been handed. Sabbatini and her co-authors say this behavior reflects a lack of inhibitory control in sight of food rather than a lack of understanding of the task’s requirements. They cite observations that capuchins have an “active and persistent foraging style and a tendency to manipulate and combine objects that is difficult to restrain in the presence of food.” Capuchins also perform worse in experiments of delay gratification than some other primates, typically waiting for 10-30 seconds as compared to 120 seconds for chimpanzees.

Thinking About Tools

Why is succeeding at this test such so significant? When the capuchins received a tool, they had to judge its suitability to meet the task’s requirements — to get into the tube of yogurt. If they judged it unsuitable (i.e., it was a rigid tool), they had to then judge whether the demonstrated tool on the platform was appropriate and if they could retrieve it using the tool in their hands.

Since the rigidity of the tools determined their usefulness, the monkeys could only judge the tool resting on the platform based on how it moved when the experimenter demonstrated it. Capuchins had to keep in their working memory the properties of the platform tool to organize their behavior. This kicks up the cognitive demands of this experiment over ones in which animals are asked to use tools that vary in a visual property like length or size. The visual information is always available to them; the flexibility of the tools has to be remembered after it is demonstrated to them.

This study adds to the research showing capuchin monkeys are adept at some of the subtleties of tool use. It’s a major component of their natural behavior in the wild, and studies like this one are beginning to unravel the cognitive processes underlying this ability.